One way of controlling escalating energy costs is by limiting energy consumption. In a modern office building, the principle energy consumers are lighting and heating and cooling. To conserve energy, the thermostat is "turned back" and the lighting is reduced during non-office hours. Reducing the energy consumption from lighting essentially involves dimming the lamps or turning off selected lamps. To conserve energy during non-office hours, most banks of lamps on a floor are turned off, with a few banks of lamps being left on to provide some lighting for security. The other approach to conserving energy consumption involves dimming the fluorescent lamps during non-office hours. As a result of being dimmed less power is consumed, while at the same time a minimum light level is maintained for security purposes.
In a typical office building the lighting system comprises banks or groups of fluorescent lamps. A fluorescent lamp is a type of lamp in which light is generated by fluorescence. The most common form of fluorescent lamp comprises a gas-discharge tube which contains a low-pressure gas such as mercury. The inner surface of the tube is coated with phosphor and when a current passes through the tube a discharge results and the ultraviolet radiation produced strikes the phosphor which then emits visible radiation. To start the discharge, i.e. turn on the lamp, the current must be provided at a sufficiently high voltage level, and typically a form of ballast circuit is utilized to produce the discharge current.
Compared to incandescent lamps, fluorescent lamps present special problems with respect to dimming. Various solutions have been proposed for dimming fluorescent lamps, including a magnetic ballast, an electronic ballast, and an electronically tapped voltage transformer.
The magnetic ballast solution produces a high voltage when there is no discharge in the lamp (i.e. the lamp is not conducting) and also feeds a "cathode heater circuit". When the arc (i.e. discharge) starts in the tube, the voltage at the output of the secondary winding on the ballast collapses to a level which is necessary to sustain the arc. The ballast absorbs, i.e. through its inductance, the excess voltage from the power source. There have been several dimmers proposed in the art based on the variation of the voltage controlling the discharge in the lamp, but none of these solutions have achieved any commercial success.
Another type of known dimmer for fluorescent lamps is based on an electronic ballast. The electronic ballast generates a rectified DC voltage from a power source and injects a resonant current into the lamp tube. The resonant current has a relatively high frequency (typically 20 kHz) and as a result special tubes are required for the fluorescent lamps. Each lamp requires an electronic ballast. The electronic ballast is modified for dimming control by providing a variable DC voltage.
In view of the shortcomings with the state of art devices, there remains a need for a dimmer for use with fluorescent and other types of gas discharge lamps.